Only natural?

Equations over Sets of Natural Numbers with Addition Only

Systems of equations of the form X = Y Z and X = C are considered, in which the unknowns are sets of natural numbers, “+” denotes pairwise sum of sets S+T = {m + n | m ∈ S, n ∈ T}, and C is an ultimately periodic constant. It is shown that such systems are computationally universal, in the sense that for every recursive (r.e., co-r.e.) set S ⊆ N there exists a system with a unique (least, great...

Natural history of ABC systems: not only transporters.

In recent years, our understanding of the functioning of ABC (ATP-binding cassette) systems has been boosted by the combination of biochemical and structural approaches. However, the origin and the distribution of ABC proteins among living organisms are difficult to understand in a phylogenetic perspective, because it is hard to discriminate orthology and paralogy, due to the existence of horiz...

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Thy-1+ and Thy-1- natural killer cells. Only Thy-1- natural killer cells suppress dendritic cells

Cells enriched for NK activity (poly I:C induced, x-ray resistant, and nonadherent), include two phenotypically and functionally different populations. Both populations of NK cells are AGM1+, Ly-1.1-, Ly-2.1-, Ia-, and have the morphology of large granular lymphocytes. One population, however, is Thy-1+ while the second population is Thy-1-. Thy-1+ NK cells lyse YAC-1 and P815 target cells; Thy...